Added connection handler for socket.io

The object, socket, which is passed to your handler, is actually the
socket of the client. Think about it as a junction between your server
and the user's browser. Upon a successful connection, we send a welcome
type of message, and, of course, bind another handler that will be used
as a receiver. As a result, the client should emit a message with the
name, send, which we will catch. Following that, we simply forward the
data sent by the user to all other sockets with io.sockets.emit.